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K324 / 2027
Experimental Chemistry

Lesson 2 of 6

Collect a gas using its properties

Solubility and density determine a suitable setup.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can design simple experimental setups.Syllabus

    Syllabus K324, 1.1(b). Design simple experimental setups

Collect a gas using its properties

Solubility and density determine a suitable setup.

A gas syringe is convenient for measuring gas volume. The bung and connections must be airtight, the plunger must move freely, and the capacity must exceed the expected volume. Begin collection promptly: gas lost before sealing makes measured volume too small.

Choose the collection method
MethodSuitable whenExample or limitation
Displacement of waterGas is insoluble or only slightly soluble in waterHydrogen or oxygen; unsuitable for very soluble ammonia.
Upright gas jar with gas entering near the bottomGas is denser than air and would dissolve in waterCarbon dioxide can displace air upwards.
Inverted gas jar with gas entering near the topGas is less dense than airAmmonia displaces air downwards.
Gas syringeGas can be collected without significant leakage or reaction with apparatusUseful for volume-time measurements; very soluble gases need an appropriate dry setup.
Pure only

To obtain a dry gas, pass it through a drying agent that removes water without reacting with the gas. The drying stage goes after generation and before dry collection. Collecting a dried gas over water would make it wet again.

Pure only
Prescribed drying agents
AgentSelection principle
Calcium oxideSuitable for ammonia; it is basic and would react with acidic gases such as carbon dioxide.
Concentrated sulfuric acidAbsorbs water, but reacts with ammonia, so cannot dry it.
Fused calcium chlorideAnhydrous salt absorbs water; unsuitable for ammonia, which it also absorbs.

Check your understanding

Why is collecting ammonia over water unsuitable?

  1. Ammonia is denser than air, so it sinks out of the gas jar before it is sealed.
  2. Ammonia reacts with the glass of the gas jar when the jar is wet with water.
  3. Ammonia is very soluble in water, so much of it dissolves instead of collecting.
Show the answer

Ammonia is very soluble in water, so much of it dissolves instead of collecting.

A very soluble gas is lost into the water. Ammonia is less dense than air, so it is collected by upward delivery into an inverted dry container.